1. Field of the Invention
The present invention relates to navigation receivers, and in particular to receivers that can build their own long-term models of the global positioning system (GPS) satellite orbits and clocks.
2. Description of the Prior Art
Satellite navigation receivers, like those found everywhere for the Global Positioning System (GPS), depend on knowing the exact orbital positions of each satellite they are tracking in order to calculate the position of the receiver. Such ephemeris information is only periodically updated and it goes stale rather quickly.
Mobile phones and digital cameras now come routinely equipped with GPS navigation receivers that provide position fixes for their users and locations for the photographs taken. These embedded GPS navigation receivers are the assisted type (A-GPS) that can download satellite ephemeris and almanac data from network servers, and thus do not have to wait the usual long times otherwise needed to collect the information directly from the satellites themselves. However, network connections are not 100% reliable nor always available, especially for mobile users. Not having the satellite ephemeris and almanac data immediately available on a cold start can mean the time to a first fix can be unacceptably long.
In the GPS system, at least twenty-four satellites circling the earth at an attitude of 20,200 km are spaced in orbit such that a minimum of six satellites are in view at any one time to a user. Each such satellite transmits an accurate time and position signal. GPS receivers measure the time delay for the signal to reach it, and the apparent receiver-satellite distance is calculated from that. Measurements like this from at least four satellites allow a GPS receiver to calculate its three-dimensional position, velocity, and system time. The apparent distance also contains the time offset of the satellite and receiver time offset from the true GPS system time.
The solution to the receiver position depends on knowing where each of the relevant satellites are in three-dimensional space and the time offset from GPS time of each satellite. The respective positions are reported as parameters belonging to a set of Keplerian equations. In conventional GPS systems, the GPS ephemeris includes all the items in Table I.
TABLE IGPS Ephemeris MessageNameSymbolReference time of ephemeristoeSquare root of semi major axis{square root over (a)}EccentricityeInclination angle (at time toe)i0Longitude of ascending nodeΩ0Argument of perigee (at time toe)ωMean anomaly (at time toe)M0Rate of change of inclination angledi/dtRate of change of Longitude of ascending node{dot over (Ω)}Mean motion correctionΔnAmplitude of cosine correction to argument of latitudeCucAmplitude of sine correction to argument of latitudeCusAmplitude of cosine correction to orbital radiusCrcAmplitude of sine correction to orbital radiusCrsAmplitude of cosine correction to inclination angleCicAmplitude of sine correction to inclination angleCis
During the period of validity of the ephemeris message, it is possible to compute the near exact position and velocity of a satellite and its clock (time) offset from GPS time at any one instant in its flight using the complete GPS ephemeris message information downloaded to the navigation receiver. This of course presumes the GPS ephemeris message can be downloaded.
The computed positions of the GPS satellites are very sensitive to small variations of most of these parameters, so it is necessary to fully communicate them during each ephemeris download. But, as described in United States patent application, US 2005/0278116 A1, published Dec. 15, 2005, it has also been observed that the two harmonic corrections to inclination angle, Cic and Cis, are not so critical. eRide, Inc. (San Francisco, Calif.), at least, communicates “compact” satellite models in which one of these two parameters are set to zero and not used in the computation of satellite position. The solution accuracy is not significantly degraded. The second order clock time offset has little impact as well and can be ignored.
It takes a GPS navigation receiver a minimum of 12.5 minutes of continuous operation to be able to collect the complete almanac and ephemeris describing all the orbiting satellites and their trajectories. There is a lot of data, and the 50-Hz modulation used to transfer this information (navData) is very slow. The signal strength at the receivers must also be good in order to be able to demodulate the navData sub-carriers. So a lack of time and/or a lack of strong signal can frustrate a user trying to get a quick first position fix.
Conventional receivers solve this problem by storing the almanac data in previous operational sessions. Thereafter, currently collected ephemeris data is compared to the stored almanac data to determine what needs to be updated. So a rather complete and updated almanac is built up and maintained at the receiver itself that is instantly available at future warm starts. Thus, after considering the acquisition time, the time to first fix of a new GPS session is dependent mainly on the time to collect the ephemeris.
Conventional satellite ephemeris data from network servers goes stale rather quickly, so A-GPS navigation receivers need to connect with their servers at least daily. The typical line-of-sight ranging accuracy degrades over twenty-five meters within four hours after the center time-of-ephemeris (toe) of the normally accepted applicability window. Currently, GPS satellites broadcast a new ephemeris every two hours, and the time-of-ephemeris is one hour in the future at the cutover to the new model. Since the GPS satellites move in twelve hours orbits, using models from a previous session will generally allow position fixes only five hours after the last session. These fixes degrade if the number of visible satellites involved drops to three or less. Given the realities of the ways mobile phones and cameras are used, connecting to the server every day may not be possible or practical.
Other extended assistance techniques have evolved beyond real-time assisted GPS techniques. Instead of waiting on the actual ephemeris information being received from the satellites flying overhead, a synthetic equivalent is predicted and pre-loaded. Such prediction information, or “extended ephemeris,” is an estimate of a satellite's future ephemeris that can be good for up to a week. When synthetic assistance is available on a device, the GPS startup times can be significantly reduced compared to real-time assistance techniques, since no server transaction is required.
Two kinds of extended ephemeris solutions are now conventional, network enabled and fully autonomous. Network enabled solutions require a periodic data download from a prediction server. Fully autonomous solutions do not need network support, they learn and generate their own synthetic assistance from their own satellite observations.
Fully autonomous solutions have sometimes inconvenient limitations, they can only predict data for the satellites they have actually seen and may require multiple observations that have close time proximity. This means the availability of extended models is dependent on how often the receiver is used and thus requires higher power consumption. With autonomous models, the data derived is usable for up to three days before the accuracy degrades too much. Network enabled solutions provide longer and more accurate predictions, sometimes up to ten days or two weeks for entire constellations.
Although extended ephemeris solutions have become commercially available, they have remained strictly proprietary to each chipset vendor who implemented the feature. Chipset proprietary network enabled solutions usually compute the prediction data on a server, then periodically download to the portable device. These server-based prediction techniques generally impose substantial weekly data payloads, typically fifty to eighty kilobytes per constellation. For applications where broadband data connectivity may be problematic or too costly, such overhead can be prohibitive.
What is needed are compact long-term models of the GPS satellite orbits and clocks that can be computed by GPS receiver with limited computing capabilities and little or no network access and do not place special requirements on the time proximity of observed ephemeris in order to generate extended models.